In recent years, downsizing of an electronic part and high density mounting of such part have been advanced rapidly in the technology for mounting an electronic part to an electronic circuit board. As a result, there has been a rapid and increasing demand for a solder material to facilitate an advanced bonding and the like of an electronic part with narrow pitching and sufficient strength. In addition, while there is a growing concern to the environment, a move to implement legal regulations for the disposal of industrial wastes such as electronic circuit board has been observed.
In the following, one example of a conventional solder material will be described referring to the drawings. FIG. 3 shows the microstructure of the conventional solder alloy and the microstructure at the bonding interface between the conventional solder material and a copper land. In FIG. 3, numeral 1 designates an .alpha.-solid solution with an Sn-rich phase. Numeral 2 designates a .beta.-solid solution with a Pb-rich phase. Numeral 3 designates an intermetallic compound whose composition is Cu.sub.3 Sn. Numeral 4 designates another intermetallic compound whose composition is Cu.sub.6 Sn.sub.5. Numeral 5 designates a Cu land.
The conventional solder as shown above used to be an eutectic alloy of Sn and Pb, which is composed of 63% by weight of Sn and 37% by weight of Pb and whose eutectic point is 183.degree. C. In addition, the alloy had a microstructure where the .alpha.-solid solution 1 and the .beta.-solid solution 2 are in the lamellar state. Moreover, the intermetallic compounds 4 and 5 were formed on the solder/copper land bonding interface.
In recent years, from the standpoint of environmental protection, it has been under rapid progress in the world to regulate the use of lead which is a poisonous substance contained in the solder material (Sn--Pb alloy). The conventional solder material had a problem that upon exposure to acid-rain of a waste of a printed circuit board with soldering of the conventional solder material, there occurs voluminous elution of the poisonous substance lead, whose toxicity induces adverse effects on the human body of neuropathy and the like. The conventional solder material had also another problem that since it has a lamellar structure in its alloy, the microstructure is thickened upon exposure to a high-temperature environment and a slip occurs on the microstructural interface of the solder upon application of a force on the solder, inducing crack development thereon. Moreover, the conventional solder material had still another problem that at soldering and under the high-temperature environment, double-layered hard and fragile intermetallic compounds will grow on the solder/copper land bonding interface, inducing crack development on the bonding interface.